Wavelength division multiplexing (WDM) has proven to be a key technology for accommodating large bandwidths for the global spread of multimedia communications in optical fiber networks. In WDM-based networks, optical add/drop multiplexer (OADM) devices used to insert (add) or extract (drop) a specific wavelength in optical fiber communication systems are essential. These components allow the extraction of a wavelength from a transmission loop and the addition of the same wavelength to the network (Reference: T. Erdogan, “Optical add-drop multiplexer based on an asymmetric bragg coupler”, Optics Communication 157, 249-264 (1998)). Numerous different architectures of add/drop filters based on optical waveguides have been demonstrated. These include Mach-Zehnder interferometer (MZI) based add/drop filters (Reference: D. Gauden, E. Goyat, C. Vaudry, P. Yvernault, and P. Pureur, “Tunable Mach-Zehnder-based add-drop multiplexer”, Electro. Letter 40, 1374-1375 (2004)), grating-assisted co-directional couplers (Reference: M. Kalishov, V. Gralsky, J. Schwartz, X. Daxhelet, and D. V. Plant, “Tunable waveguide transmission gratings based on active gain control”, IEEE Journal of Quantum Electro. 40, 1715-1724 (2004)), asymmetric Bragg coupler (ABC) based filters (Reference: T. Erdogan, “Optical add-drop multiplexer based on an asymmetric bragg coupler”, Optics Communication 157, 249-264 (1998)), and Bragg reflector channel waveguide filters (Reference: M. Dainese, M. Swillo, L. Wosinslki, and L. Thylen, “Directional coupler wavelength selective filter based on dispersive bragg reflection waveguide”, Optics Communication 260, 514-521 (2006)). MZI-based add/drop filters yield excellent insertion loss and channel isolation, and they can be built on both all-fiber and integrated-optics platforms. However, their performance is extremely sensitive to the balance of the interferometer and relative placement of the two gratings; therefore, some post-fabrication trimming is often necessary. A grating-assisted co-directional coupler, consisting of two dissimilar waveguides and a long period grating, has been widely discussed for use as a wavelength filter. It has the advantage of a long grating period (about a few tens of micrometers), facilitates the fabrication by using standard photolithography, and has a low back-reflection characteristic, avoiding unwanted optical resonances. The main drawback of such a device is that when it operates in a small spectral bandwidth, a long interaction grating length (about a few hundreds of grating periods) is required. Therefore, it is not beneficial for device integration. Bragg reflector channel waveguide filters have excellent return loss and crosstalk characteristics and are inherently very stable. However, the need for non-reciprocal optical circulators limits its application in integrated optical formats. ABC-based filters, which operate in a contra-directional mode, are not sensitive to grating placement for obtaining a desired filter spectrum. Therefore, they have better stability and reproducible mass production than MZI-based filters.
Polymeric materials offer a conceivable platform to fabricate complex yet affordable integrated optical devices, especially dense wavelength division multiplexers, on a planar substrate; this is due to the benefits of low production cost, easy processing, and mechanical flexibility. Polymer surface-relief Bragg grating, which provides a narrow bandwidth, low crosstalk, and flat-top pass band, has become an essential component for various applications in optical communications and optical sensing. For example, Butler et al used polymer surface-relief Bragg grating on an integrated optical waveguide structure to fabricate a chemical sensor (Reference: T. M. Bulter, E. Igata, S. J. Sheard, and N. Blackie, “Integrated optical Bragg-grating-based chemical sensor on a curved input edge waveguide structure,” Opt. Lett., 24, 525-527 (1999)). Noh et al demonstrated a cost-effective tunable wavelength laser based on the thermo-optic tuning of a polymer waveguide Bragg reflector for WDM optical communications (Reference: Y. O. Noh, H. J. Lee, J. J. Ju, M. S. Kim, S. H. Oh, and M. C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16, 18194-18201 (2008)). Other applications of tunable lasers and filters were demonstrated in the references: G J., J. H. Lee, M. Y. Park, C. Y. Kim, S. H. Cho, W. Lee, and B. W. Kim, “Over 26-nm Wavelength Tunable External Cavity Laser Based on Polymer Waveguide Platforms for WDM Access Networks,” IEEE Photonics Technol. Lett. 18, 2102-2104 (2006); J. H. Lee, M. Y. Park, C. Y. Kim, S. H. Cho, W. Lee, G J., and B. W. Kim, “Tunable External Cavity Laser Based on Polymer Waveguide Platform for WDM Access Network,” IEEE Photonics Technol. Lett. 17, 1956-1958 (2005); M. C. Oh, H. J. Lee, M. H. Lee, J. H. Ahn, S. G Han, and H. G. Kim, “Tunable wavelength filters with Bragg gratings in polymer waveguides,” Applied Physics letters 73, 2543-2545 (1998).
The inventors of the present invention recently demonstrated a process to rapidly produce submicron range gratings on waveguide for optical filters using soft lithography, micro-molding, and holographic interference techniques. In this method, the grating structure on a polymer is first fabricated using holographic interferometry and the micro-molding processes. Polymeric wavelength filters are produced by a two-step molding process where the master mold is first formed on a negative tone photo-resist and subsequently transferred to a PDMS mold; following this step, the PDMS silicon rubber mold is used as a stamp to transfer the pattern of the polymeric wavelength filters onto a UV cure epoxy. A high aspect ratio and vertical waveguide sidewalls are obtained by this method, and consistent reproduction of the grating on a UV polymer has been achieved with this process (Reference: W. C. Chuang, C. T. Ho and W. C. Wang, “Fabrication of a high resolution periodical structure using a replication process” Opt. Express 13, 6685-6692 (2005); W. C. Chuang, C. K. Chao and C. T. Ho, “Fabrication of a high resolution periodical structure on polymer waveguide using a replication process” Opt. Express 15, 8649-8659 (2007)). In the present invention, we describe a technique that combines the holographic interferometry, soft lithography, and a simple replication processes for fabricating a polymeric ABC.
Polymeric ABC filters were constructed using the planar channel waveguide configuration. A pair of parallel channel waveguides with different widths was embedded into a planar substrate (Referring to FIGS. 3(a) and 3(b)). The two waveguides are asynchronous because the effective refractive indices of the two waveguides are quite different. In spite of the large index mismatch between the two waveguides, an efficient power coupling was achieved using the Bragg grating engraved on the bottom of the two waveguides. The maximum cross-reflection power coupling occurred at a specific wavelength λd1 (Bragg wavelength) satisfying the Bragg reflection condition, (neff1+neff2)Λ=λd1, where neff1 and neff2 are the effective indices of the two waveguide modes and Λ is the grating period. It implies that the center wavelength of the ABC filter is proportional to the sum of the effective indices of the two individual waveguides. Therefore, when the effective index of any individual waveguide was changed it results in a shift in the center wavelength. Furthermore, an unwanted reflection wavelength, denoted by self-reflection Bragg wavelength (λd2), caused by the grating of input waveguide is occurred in the input end. The self-reflection light results in broadening the transmission spectrum of the filters if its spectrum overlaps with the spectrum of cross-reflection light. Such unwanted reflection light can be eliminated by suppressing the grating depth of the input waveguide. Another method is to make the two decoupled waveguides quite dissimilar to avoid the spectrum overlapping.